The mRNA content, with particular reference to possible differential distributions of types or titers, of isolated blastomeres from 16-cell stage sea urchin embryos will be assayed by two alternative methods,- translation in a linear and optimized cell-free wheat germ system, and addition/competition hybridization of the RNA's with DNA prefractionated to separate repeated sequences from unique ones. The same approach will be taken with RNA prepared from blastomeres of non- transcribing (Actinomycin-treated) embryos, this presumably representing the "maternal" mRNA population inherited from oogenesis. By a prior separation of postribosomal and polysomal RNP particles according to size, histone mRNAs and tubulin mRNAs will be purified from contaminating background of translatable polynucleotides and these will then be assayed, again by translation and hybridization. Patterns of transcription of these messages after fertilization, and of translation of the new and preformed messages, will be examined in the several blastomere types with and without reaggregation permitted, in order to assess the possible role of cell-cell interaction in switches of gene expression. Of particular interest among the latter will be the fl sub m - fl sub g (lysine-rich histone of morulae and gastrulae, respectively) switch that occurs normally in the embryos of three species of sea urchins.